1. Field of the Invention
This invention relates to an active material for battery, a method of manufacturing the active material for battery, a non-aqueous electrolyte battery and a battery pack.
2. Description of the Related Art
Since the non-aqueous electrolyte battery is designed such that the charging/discharging thereof is effected through the movement of lithium ions between a negative electrode and a positive electrode, it is expected to be useful as a battery exhibiting high energy density, so that the research and development of the non-aqueous electrolyte battery are now intensively conducted.
The non-aqueous electrolyte battery is demanded to have various properties depending on the end-use thereof. For example, if it is desired to be used as a power source for a digital camera, the battery is required to exhibit the discharging of about 3 C. Whereas, if it is desired to be used as a power source for vehicles such as a hybrid electric motor car, the battery is expected to require the discharging of about 10 C or more. For this reason, the non-aqueous electrolyte battery to be used for these end-uses is desired to exhibit especially large current characteristics.
In the non-aqueous electrolyte battery available in the market, lithium-transition metal composite oxides are employed as an active material of the positive electrode and carbonaceous materials are employed as an active material of the negative electrode. As for the transition metals included in the lithium-transition metal composite oxides, metals such as Co, Mn, Ni, etc., are generally employed.
Recently, it is studied to employ, as an active material of the negative electrode, lithium titanium composite oxides which are higher in electric potential in the absorption/desorption of lithium as compared with carbonaceous materials. The lithium titanium composite oxides are known to exhibit very small changes in volume as they are subjected to charging/discharging. For this reason, a non-aqueous electrolyte battery where the lithium titanium composite oxides are employed as an active material of the negative electrode is enabled to exhibit excellent cycle characteristics.
Among the lithium titanium composite oxides, spinel type lithium titanate (composition formula: Li4+wTi5O12 (0≤w≤3)) is especially excellent in cycle characteristics, so that this compound is considered promising as a long-life active material for battery. The spinel type lithium titanate however is relatively low in electronic conductivity and in lithium ion conductivity, so that the large current characteristics of battery using this spinel type lithium titanate are relatively low as compared with the battery using a carbonaceous material as an active material of the negative electrode.
Under the circumstances described above, JP-A 9-199179 (KOKAI) discloses a method of improving the large current characteristics of battery, wherein a carbonaceous material such as carbon black is added as a conductive material to the lithium titanium composite oxides to be employed as an active material of the negative electrode, thereby enhancing the electronic conductivity of lithium titanium composite oxides. JP-A 2002-100354 (KOKAI) also discloses a method of improving the large current characteristics of battery, which is effected through the pulverization of the lithium titanium composite oxides to shorten the diffusion path of lithium ions.
JP-A 2005-332684 (KOKAI) filed previously by the present applicant also discloses a method of improving the high-temperature cycle characteristics of a battery, which is effected through the addition of nonstoichiometric titanium oxide exhibiting conductivity to lithium titanium composite oxide to be employed as an active material of the negative electrode.
The aforementioned three patent documents however are accompanied with the following problems.
The carbonaceous material to be employed as a conductive agent is incapable of forming a film which is stable at a working electric potential of lithium titanium composite oxide, i.e., 1-2V (vs. Li/Li+). As a result, when a large quantity of carbonaceous material is added to lithium titanium composite oxides as proposed by JP-A 9-199179 (KOKAI), the carbonaceous material is permitted to excessively react with an electrolyte to generate gas and, when the electrolyte is exhausted, the battery is caused to deform, resulting in the deterioration in performance of battery.
When the pulverized lithium titanium composite oxides are to be employed as suggested in JP-A 2002-100354 (KOKAI), it is required to incorporate a large quantity of carbonaceous material in order to form a conductive path linking these pulverized particles, resulting in the generation of a large quantity of gas. Further, on the occasion of manufacturing a negative electrode by a process wherein a slurry containing fine particles of lithium titanium composite oxide is coated on the surface of a current collector and dried to form a negative electrode layer containing an active material, it is required to incorporate a large quantity of binder into the slurry so as to immobilize the fine particles of lithium titanium composite oxide in the negative electrode layer. As a result, the viscosity of slurry becomes higher, making it difficult to coat the slurry on the current collector and, at the same time, the binder itself becomes an obstacle for the diffusion of electrons and ions, thereby deteriorating the large current characteristics.
On the other hand, the employment of nonstoichiometric titanium oxide exhibiting conductivity as proposed by JP-A 2005-332684 (KOKAI) is effective in overcoming the problem to be caused by the addition of a carbonaceous material as a conductive agent as described in JP-A 9-199179 (KOKAI). However, although the non-aqueous electrolyte battery equipped with a negative electrode containing lithium titanium composite oxide and nonstoichiometric titanium oxide is effective in improving high-temperature cycle characteristics, it does not necessarily indicate excellent large current characteristics. There are two reasons for this problem. One reason is that since the lithium titanium composite oxide and the nonstoichiometric titanium oxide are both granular, they are contacted with each other simply through point contact. The other reason is that it is difficult to make the nonstoichiometric titanium oxide into fine powder as compared with the carbonaceous material.